Method and device for the compression and decompression of data in a transmission system

ABSTRACT

A method for the compression of data applied to a system for the transmission of data in protocol data units between at least one emitter terminal and at least one receiver terminal. Each protocol data unit comprises a header field and a data field. A first sequence of N source protocol data units is associated with a second sequence of M protocol data units to be transmitted, M being smaller or equal to N. The data fields of the protocol data units transmitted comprise data that has been compressed (according to at least one compression algorithm). The data fields are then made up of data contained in the data fields that has been compressed, and also data allowing reconstruction of N source protocol data units.

BACKGROUND OF THE INVENTION

The field of the invention is that of data compression systems and, morespecifically, that of structured transmission systems wherein thecommunications between two terminals are achieved by exchanges betweenprotocol data units or PDUs.

The invention can be applied notably to packet transmission systems(systems implementing a layer 3 procedure according to the seven-layerOSI standardization model), but also to frame transmission systems(layer 2) or systems using transport protocol data units (layer 4). Moregenerally, the invention can be applied to any type of transmissionsystem, whatever the layer (according to the terminology of the OSIstandards) of the communications protocol associated with the systemconsidered.

One particular field of application of the invention is that of dataexchange systems working according to the X25 protocol standardized bythe CCITT. On French territory, the invention can be applied thereforeto communications made through the public TRANSPAC (registered mark)network.

The invention relates more particularly to the optimization ofcommunications, i.e. notably the reduction of costs through thereduction of volumes of data transmitted and/or of communication times.

It is indeed known that, especially when a public communications networkneeds to be used, the costs generally depend inter alia on the number ofprotocol data units (or subsets of these units) transmitted and on thecommunication times.

A known way of reducing the volume of the data transmitted is tocompress the data to be transmitted.

Many data compression techniques are indeed known, notably forpoint-to-point links. Thus for example an adaptive compression algorithmcalled the Ziv-Lempel algorithm has been chosen in the CCITTrecommendation V42a for asynchronous modems.

However, in the case of systems working by exchanges of protocol dataunits, for example according to the x25 standard, several difficultiesare generally encountered.

Thus, there is a known system, called DATAMISER (registered mark), thatmethodically carries out the byte-by-byte compression of the entire flowof data delivered by a terminal. When this flow of data is organized inprotocol data units, each comprising zones descriptive of the exchange(header fields) and data zones (data fields), a compression system suchas this does not comply with this form of structuring. In other words,it neither recognizes nor keeps the headers. It therefore cannot be usedfor an exchange going through a packet-switched network.

In order to overcome this drawback, the selective compression of thedata zones might be considered. In this case, the headers are preservedintegrally, and the quantity of information elements to be transmittedis substantially reduced. In other words, with a source protocol dataunit, there is associated a compressed protocol data unit whose headeris identical to that of the source protocol data unit.

In practice, it turns out that the efficiency of this technique is notoptimal. Indeed, the number of packets (or PDUs) transmitted remains bydefinition the same. The gain is therefore minimal since partially emptypackets are transmitted.

The invention is notably aimed at overcoming these various drawbacks ofthe prior art.

More specifically, an aim of the invention is to provide a method tooptimize the efficiency of communications made on a data exchangenetwork, reducing the number of protocol data units or subsets of unitstransmitted.

Another aim of the invention is to provide a method such as this thatrelates to the structure of the protocol data units transmitted. Inother words, an aim of the invention is to provide a method such as thisthat is transparent from the viewpoint of the terminals as well as fromthat of the transmission network.

More generally, the invention is aimed at providing a method such asthis that enables the reduction of at least one of the followingparameters:

cost of the call;

duration of the call;

volume of traffic generated by the network (in terms of number of PDUsand/or bytes) without losing any element of the information transmitted.

Another aim of the invention is to provide a method such as this thatcan be implemented on any structured system of data transmission.

The invention is also aimed at providing a device, implementing a methodsuch as this, that can be inserted between a terminal and acommunications network transparently, i.e. without requiring anyadaptation either of the terminal or of the network.

These aims as well as others that shall appear hereinafter are achieved,according to the invention, by means of a data compression method of thetype applied to a system of data transmission by the exchange ofprotocol data units between at least one transmitter terminal and atleast one receiver terminal, each of said protocol data units comprisinga header field and a data field, said method associating, with a firstsequence of N source protocol data units, a second sequence of Mprotocol data units to be transmitted, M being smaller than or equal toN, at least certain of the data fields of said protocol data units to betransmitted comprising data corresponding to the compression, accordingto at least one compression algorithm, of data contained in the datafields of at least two of said source protocol data units, and at leastcertain of said protocol units to be transmitted comprisingreconstruction data enabling the reconstruction of said first sequenceof source protocol data units from said second sequence of protocol dataunits to be transmitted.

In this way, the efficiency of the call may be greatly increased but theinitial structure of the data is preserved. It is indeed possible, atreception, to reconstitute the source sequence by means of thereconstruction data added to the transmitted data.

SUMMARY OF THE INVENTION

According to the invention, the structure of the data is modifiedbriefly during the transmission. This enables a reduction in the numberof protocol data units to be transmitted but there is provision for anew step, additional to that of the known techniques, namely theinsertion, among the pieces of compressed data to be transmitted; ofreconstruction data.

A method of this kind advantageously comprises the steps of:

the reception of said first sequence of source protocol data units,

data compression associating, with each of said protocol data units, acompressed data sub-field comprising data corresponding to thecompression of the data of the data field of said source protocol dataunit,

the generation of data for the reconstruction of said source protocolunits,

the concatenation of said reconstruction data and said compressed datasub-fields in a sequence of concatenated data,

the slicing of said sequence of concatenated data into data blockshaving a predetermined length chosen from a sub-set of at least onepossible length,

the creation of said second sequence of protocol data units to betransmitted, the data field of each of said protocol data units to betransmitted comprising at least one of said data blocks.

According to this method therefore, a sub-protocol of the network istherefore added, so to speak, transparently (for this network). It mustbe noted that the steps of concatenation, slicing and creation of thesecond sequence, which have been highlighted here solely in order tomake it easier to understand the invention, may actually be donesimultaneously (the creation of a packet is begun as soon as the piecesof data to be concatenated are received).

In practice, the steps of concatenation and slicing are generallycarried out simultaneously: full packets are formed by concatenation andthe packet is sent out as soon as it is full. If a data sub-field hasbeen sliced (to fill up the data field of a packet to be transmitted),the remaining part of this sub-field forms the start of the data fieldof the next packet to be transmitted.

Advantageously, said step for the generation of reconstruction datacomprises a step of association (26), with each of said compressed datasub-fields (30), of a sub-header (29) comprising at least oneinformation element (27) representing the length of said compressed datasub-field (30).

According to other advantageous embodiments of the invention, said stepfor the generation of reconstruction data may comprise:

a step for the association (26), with each of said compressed datasub-fields (30), of a sub-header (29) comprising at least one designatordesignating the position of the end of said sub-field in the data fieldof the corresponding protocol unit to be transmitted (51_(i));

a step to generate a marker having a predetermined content, known tosaid receiver terminal;

a step for the association, with each protocol data unit to betransmitted (51_(i)), of a descriptor of the content of the data fieldof said protocol data unit.

Preferably, said data enabling the reconstruction of said first sequenceof source protocol data units include at least one of the informationelements present in the header field of each of said source protocoldata units.

Indeed, apart from the length of the compressed data, it may benecessary, in order to reconstitute the source sequence, to keepinformation elements that qualify the source protocol data units.

In a particular embodiment of the invention, said sub-header comprisesat least one first byte bearing at least one of the information elementspresent in the header field of the source protocol data unitcorresponding to said sub-header, and at least one second byte bearingthe length of the associated compressed data sub-field.

Advantageously, said compression algorithm is an adaptive compressionalgorithm managing dictionaries of series of data at transmission and atreception, said dictionaries being updated identically at eachtransmission of one of said protocol data units.

Indeed, this adaptive algorithm proves to be especially promising interms of compression efficiency.

Preferably, the method includes a data compression step comprising thesteps of:

implementing a compression algorithm that associates a compressed datafield with a source data field;

comparing the length of said source data field and compressed data fieldand selecting the shortest data field as the compressed data sub-field.

It is clear that if the compression is not efficient, it will bepreferable to transmit the data in a non-compressed form.

In this case, said sub-header has an information element representingsaid selection of the source data field or of the compressed data field.

The protocol data units considered may be of any type, for example thetype comprising data packets, data frames or layer 4 (transport)protocol data units.

In a standard way, these protocol data units may have a protocol dataunit length chosen from a set comprising at least one possible length.

Advantageously, the method also includes a step for the initializing ofa transmission, consisting notably in ascertaining that the receiver hasmeans available to implement a decompression method corresponding tosaid compression method.

The term "receiver" is understood to mean terminals equipped internallywith compression/decompression means or specificcompression/decompression devices associated with terminals.

The invention can therefore be used to transmit only full data unitssince their data fields include concatenated data belonging to severalsource units. However, the filling of a unit should not be an obligatorycondition, otherwise there is a risk that the method may be inhibited.Indeed, especially when an end of a sequence is concerned, it should bepossible to send out an incomplete data unit. To this end, two timedelays may be envisaged.

Thus, the method advantageously comprises a first time-delay step fixinga maximum waiting period D1 before the creation of a non-full protocoldata unit to be transmitted.

Furthermore, it may also include a second time-delay step, fixing amaximum waiting period D2, D2 being smaller than D1, before the creationof a protocol data unit to be transmitted comprising at least one fulldata block of a predetermined length and no data block that is not full.

The invention relates naturally to a decompression method that issymmetrical with a compression method described here above, comprisingthe steps of:

searching for and analyzing said data enabling the reconstruction ofsaid first sequence of protocol data units,

the decompression of said data corresponding, in compressed form, to thedata contained in the data fields of the source protocol data units;

the restitution of said first sequence of source protocol data units.

It also relates to any device implementing this method of compressionand/or this method of decompression and notably a device comprising,firstly, means of connection to at least one transmitter/receiverterminal, behaving similarly to the data transmission system and,secondly, means of connection to said data transmission system behavingsimilarly to a transmitter/receiver terminal.

In this way, the device is quite transparent.

Other features and advantages of the invention shall appear from thefollowing description of a preferred embodiment of the invention, givenby way of a non-restrictive illustration, and from the appended drawingsof which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a data transmission system comprisingtwo devices according to the invention;

FIG. 2 is a block diagram illustrating the compression method of theinvention;

FIG. 3 illustrates the processing undergone by a source packet sequenceaccording to the method of FIG. 2;

FIG. 4 exemplifies a sub-header that can be used in the compressedpackets of FIG. 3;

FIG. 5 is a simplified drawing of a device implementing the method ofFIG. 2.

FIG. 6 is a simplified drawing of a device implementing thedecompression and reconstruction method and is symmetric to FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment described here below can be applied notably tocommunications networks providing an interface according to the standardx25, and especially to networks using a segment-based pricing systemsuch as the French public network TRANSPAC (registered mark). It isclear however that the invention is not restricted to networks of thistype but may be applied, on the contrary, in every case where theexchange of data is clone by protocol data units.

Thus, although hereinafter it is more specifically the case of datapackets that will be considered, the invention can be implemented atevery layer of the standardization protocol OSI, notably at the layer 2and/or at the layer 4.

As stated further above, an aim of the invention therefore is to improvethe efficiency of the communications on a given network while at thesame time preserving the structure of the data train transmitted.

FIG. 1 exemplifies an implementation of the invention. Threetransmitter/receiver terminals 11_(A), 11_(B) and 11_(C) are connectedto a packet communications network 12.

These terminals 11_(A) and 11_(C) represent any end system that can getconnected to a network such as for example a computer, a data basecenter, a data acquisition terminal, etc.

Two terminals 11_(A) and 11_(C) can be associated for a particular call.These two terminals can, if necessary, manage several callssimultaneously. For example, the terminal 11_(A) can handle a first callwith the terminal 11_(B) at the same time as a second call with theterminal 11_(C).

In the preferred embodiment of the invention described in relation withFIG. 1, the compression/decompression means are installed in devices13_(A) and 13_(B) that are independent of the terminals 11_(A) and11_(B). These compression/decompression devices are therefore splicedinto the connection cable to the network and are transparent both forthe terminal 11_(A) and for the network 12. In other words, seen fromthe terminal 11_(A), the device 13_(A) behaves (14, 15) like the networkand, seen from the network 12, it behaves like a terminal (16, 17).

Each device 13_(A), 13_(B) includes data compression means 18_(A),18_(B) implementing the method that shall be described hereinafter andsymmetrical decompression means 19_(A), 19_(B). Hereinafter, it isessentially the compression method that shall be described. Thedecompression method can easily be deduced therefrom by symmetry.

It must be noted that the compression of the data of a call is not donesystematically. First of all, the receiver terminal must be associatedwith (or must include) a decompression device. Thus, any callimplicating the terminal 11_(C) will be made without compression.Furthermore, the two compression/decompression devices 13_(A) and 13_(B)must have necessary means (processing and/or memory means). It ispossible, for example, to provide for devices that are capable ofmanaging several calls, only a limited number of them being calls withcompression/decompression.

Furthermore, it is quite possible to envisage a case where thecompression is implemented only in one of the two directions of thecall, depending on the means available and/or on the usefulness of thecompression, in terms of efficiency, in each direction of communication.

Finally, although the devices 13_(A) and 13_(B) are represented hereinin the form of independent units, this independence having the majoradvantage of enabling their use with all types of existing terminals (itis enough to interconnect the device 13_(A) between the terminal 11_(A)and the network 12), it is clear that the same compression/decompressionmeans may be integrated with a terminal.

FIG. 2 is a simplified block diagram of the compression method of theinvention.

At the beginning of each call, it is possible, first of all (if this isnecessary), to carry out an initialization phase 21 consisting notablyin ascertaining that the receiver device has correspondingcompression/decompression means available.

This phase 21 comprises first of all a check on whether the targetterminal has a compression/decompression device available. This can bedone either passively, with each compression device managing a list ofterminals capable of carrying out the decompression, or actively bymeans of an interactive interrogation procedure. In a second stage, itmust be ascertained that the receiver device has decompression meansavailable.

First of all (if the compression is possible), a compression phaseproper 22 is undertaken. This phase 22 includes a step 23 for thereception of the source packets to be compacted. It must be noted thatthe method of the invention makes no assumption on the sequence ofpackets received nor on the constitution of these packets. These packetsmay or may not be full and may have a fixed or variable size, originatefrom a variety of sources etc. The method of the invention is completelytransparent with respect to the original structure.

By contrast, as shall be seen hereinafter, it ensure that this originalstructure will be preserved.

It has already been explained in the preamble that it is not possible tocompress the data fields and the headers without distinction. Accordingto the invention, only the data field 24 undergoes a compression 25. Aparticular advantageous method of compression is the one used in acompression algorithm of the so-called "Ziv-Lempel" type. The basicprinciples of encoding/decoding and updating the dictionaries aresimilar to those described in the recommendation V42a. Of course, anyother compression algorithm may be used. If necessary, severalalgorithms may be implemented simultaneously, the most efficient resultbeing the only one preserved.

Furthermore, a sub-header or information zone is created (26), thissub-header or information zone comprising notably an information element27 representing the length of the compressed data field delivered by thecompression step 25. This header furthermore has the data 28 needed toreconstruct the original sequence of packets, i.e. especially theinformation elements indicating, in the header of each original packet(non-compressed), the type of packet (qualifying information), thestructuring in complete messages according to the protocol x25, etc. Aparticular example of a sub-header is described further below withreference to FIG. 4.

The sub-header 29 and the compressed data field 30 corresponding to eachsource packet are then concatenated (31) in a continuous sequence 32 ofdata. This sequence 32 is then sliced (33) arbitrarily, i.e. withoutregard to the original packets, to a length corresponding to a datafield length permitted by the network or to a length that is asub-multiple of this permitted length.

Each block 34 thus sliced off is used for the creation (35) of a newpacket of which it becomes the data field (or a portion of this field).This new packet on the contrary has a new header that is proper to it.

The efficiency of this method will be perceived more clearly in theexemplary compression shown in FIG. 3.

A source sequence of three consecutive packets 41_(A), 41_(B) and 41_(C)is considered. Each packet is constituted in a standard way by a header42_(A), 42_(B) and 42_(C) (in the example considered, according to thestandard x25) and a data field 43_(A), 43_(B) and 43_(C).

The example considered here is that of full packets (however this is inno way an obligatory condition) having a format in accordance with theone permitted by the communications network.

Each packet 41_(i) is therefore then compressed (44). Thus, a sequenceof compressed packets 45_(A), 45_(B), 45_(C) is obtained. Naturally, thenumber of packets is preserved and the headers 42_(A) to 42_(C) of eachof them remains unchanged. By contrast, the length of the data fields46_(A), 46_(B) and 46_(C) is substantially reduced.

This compression proves to be valuable, especially when the network usesa segment-based pricing system.

This is notably the case of the TRANSPAC network which uses 64-bytesegments for its pricing. This slicing into segments, which is differentfrom the packet structure, is basically aimed at encouraging users tosend out full data packets. Indeed, each segment begun (namely eachsegment containing at least one byte) is accounted for, and thereforehas a cost price identical to that of a full segment (64 bytes).

Thus, the packet 41_(A), if sent, will be entered in the accounts fortwo segments 47₁ and 47₂. On the contrary, the packet 45_(B) has onlyone segment 48₁ entered in the accounts. In this example, thecompression enables a 50% reduction of the cost of the transmission ofthe packet.

However, a compression of this kind gives, at best, a compression rateof two (a 50% reduction of the volume of transmitted data), while thecompression techniques often make it possible to obtain compressionrates equal to 3, 4 or even more. Indeed, whatever the quality of thecompression achieved, there will always be at least one segment invoicedper packet.

The invention proposes a novel and highly efficient approach to thisproblem. Indeed, after the compression 44, a complementary compactionprocessing step 50 is carried out.

According to the invention, a new sequence of packets 51_(D), 51_(E) iscreated. These packets are quite independent of the packets of thesequence 41_(A) to 41_(C). In particular, their headers 52_(D), 52_(E)correspond to none of the headers 42_(A) to 42_(C).

By contrast, the set of useful data needed for the reconstruction of thesource packet train is available in the data fields 53_(D) and 53_(E) ofthe new packets 51_(D) and 51_(E).

More specifically, and this is the major characteristic of theinvention, a data field 53_(D) does not contain solely the concatenateddata of the data fields 46_(A) and 46_(B) but also, in the form of asub-header 47_(A), data for the reconstruction of the original sequence.

This header 47_(A) is transparent from the viewpoint of the network. Bycontrast, at decompression, firstly it enables the extraction, from thedata field 53_(D), of the sequence of data 46_(A) corresponding to thepacket 45_(A), by the information on length 54. Secondly, it contain theinformation elements needed for reconstruction, at least partially, ofthe header 42_(A).

The data fields 53_(D) and 53_(E) are considered continuously. Thus, thecompression data 46_(B) may be distributed without difficulty into twosub-sets 46_(B),1 and 46_(B),2 in two distinct packets. The sub-header47_(B) indicates the end of the sub-set 46_(B),2. A sub-header may alsobe distributed over two packets.

FIG. 4 exemplifies a sub-header that can advantageously be used withinthe framework of the protocol x25.

This sub-header is constituted by two bytes 61 and 62. The first bytecontains, firstly, two bits Q and M which are the direct reproduction ofthe bits Q (Q-bit) and M (M-bit) existing in the header of a packetaccording to the protocol x25. These bits are necessary, respectively,in order to qualify the packets and indicate the start or the end of amessage.

Furthermore, it includes a bit C which indicates whether the compressionhas been implemented for the packet considered.

Five bits (63) are available for future developments. For example, theymay carry an information element indicating the chosen mode ofcompression if several modes can be used.

The second byte comprises the length L of the sequence of compresseddata on 8 bits (54, FIG. 3). This length, if necessary, may be encodedon two bytes or more.

FIG. 5 shows a block diagram of a device according to the invention, asshown (13_(A), 13_(B)) in FIG. 1, implementing the method described hereabove.

The source packets 71 are received in a reception and separation module72 that delivers, firstly, the data field 73 and, secondly, the header74 of each packet.

The data field 73 is then compressed by the compression module 75 which,for example, implements an adaptive Ziv-Lempel algorithm. In this case,a dictionary is permanently updated in a memory 76.

The compressed data field 77 is then transmitted to a comparison module78 which selects the shortest data field 79 (comparison between thecompressed field 77 and the non-compressed field 73 and selection of theshortest field and, if the lengths are equal, the selection of thenon-compressed field, which will require less processing at reception).

Should several compression means be implemented in parallel, the module78 nevertheless carries out the most efficient selection (namely that ofthe compression mode offering the best compression rate). If necessary,the selection may be done for each packet.

A module 80 for the construction of the sub-headers 81 prepares eachsub-header (for example according to the format of FIG. 4) by takingaccount of the header 74 (to extract the bits Q and M) and informationelements relating to length L and activation of the compression Cdelivered by the module 78.

These sub-headers 81 and compressed data fields 79 are then concatenatedand sliced to the size of the segments (or packets) (82) so as to createfull segments (and more generally full packets).

In practice, this operation of concatenation and slicing is donecontinuously. The pieces of data are compressed and concatenated. Assoon as the concatenation reaches the length of a packet, this fullpacket 83 is delivered to a transmission module 84 which takesresponsibility for transmitting it to the addressee terminal.

Thus, only full packets are transmitted. This principle, however, mayintroduce a certain delay in the transmission of a source packet (aperiod of waiting for the packet to be transmitted to be full). A delaymanagement mechanism is therefore advantageously set up.

Such a mechanism is also necessary at the end of a source sequence (endof message). Indeed, it is unlikely that the last packet will be fullbut it should of course be transmitted all the same. Two time delays D1and D2 are therefore taken into account as follows:

D1: full segment time delay: a non-full packet is sent but this packethas only full segments (this time delay is notably useful in the case ofthe TRANSPAC network where the invoicing is done by segments);

D2: maximum time delay: transmission of a packet containing a lastnon-full segment (this time delay is necessary to prevent the devicefrom being inhibited, notably at the end of a source sequence).

Preferably, D2 will be chosen so as to be far smaller than D1, so as tominimize the number of segments sent.

FIG. 6 shows a block diagram of a device in the address terminalimplementing the symmetrical decompression method for decompressing thetransmitted packets. The decompression device includes a receptionmodule 86 to receive the packets, a division module 88 to dividecompression data from the sub-headers, a decompression module 90 todecompress the compressed data, and a reconstruction module 92 toreconstruct the source packets.

The efficiency of the method and of the device of the invention can beclearly seen in FIG. 3, since it is observed that two packets (51_(E),51_(D)) are transmitted while the source sequence has three full packets(41_(A), 41_(B), 41_(C)).

This efficiency is of course even more patently clear when the sourcepackets are not full. In this case, the principle of the concatenationenables a substantial gain in the number of packets (or of segments,namely of portions of packets) transmitted.

In the embodiment described here above, the method of the invention isimplanted in dedicated compression units inserted between a terminal andthe network. A dedicated compression unit such as this may be, forexample, an apparatus of the COMPLYS range (registered mark) marketed bythe firm OST (registered mark).

This method can also be directly integrated into a terminal such as, forexample, a terminal incorporating an x25 PCSNET communications card(registered mark) also marketed by OST.

It may also be integrated into a communications network element and, forexample, into an ECOM25 (registered mark) by OST.

Furthermore, as mentioned further above, the method of the invention isnot limited to the x25 networks. Indeed, it may be implemented on anyprotocol layer providing a service in connected mode (namely having areliable logic connection having neither losses nor duplication nordesequencing). For example, it may be implemented in local networkterminals such as those using the standard TCP/IP (the compression beingthen implemented at the layer 4 (TCP) or above). The terminal maynotably be a terminal equipped with a MAGELLAN (registered mark) card.

The embodiment that has been presented proposes the introduction of thesub-headers indicating the following length of the data sub-field. It isclear however that the invention can be implemented in many other waysprovided that the packets (or PDUs) transmitted convey informationenabling the reconstruction of the source packets (or PDUs).

For example, it is not obligatory for a sub-header to be insertedbetween each data sub-field and the next one. It is enough for them toappear regularly (for example every five sub-fields), each sub-headerthen bearing information elements that enable the extraction of several(for example five) consecutive data sub-fields.

Furthermore, the information on the length of the data sub-field orsub-fields may be replaced by a designator indicating the absoluteposition (and no longer the relative position) of the end of thesub-field in the transmitted packet.

According to another mode of implementation, it is also possible,between each data sub-field and the next one, to insert a data blockhaving a predetermined format (or marker) known to the receiverterminal. When it detects this marker, the terminal knows that a newsource packet is beginning.

It is also possible, at the start of each data field of the packets tobe transmitted, to provide for a zone describing the contents of thepacket (for example describing the locations of the changes of sourcepackets). A zone such as this (or descriptor) corresponds in other wordsto an extension of the header of the packets to be transmitted.

What is claimed is:
 1. A data compression or decompression method of thetype applied to a system of data transmission by exchanging protocoldata units between at least one transmitter terminal and at least onereceiver terminal, each of said protocol data units comprising a headerfield and a data field, the compression method comprising the stepsof:associating with a first sequence of N source protocol data units, asecond sequence of M protocol data units to be transmitted, M beingsmaller than or equal to N; associating with at least certain datafields of said protocol data units to be transmitted, data thatcorrespond to the compression of data contained in the data fields of atleast two of said source protocol data units, the compression beingaccording to at least one compression algorithm; associating with atleast certain of said protocol units to be transmitted, reconstructiondata that enables the reconstruction of said first sequence of sourceprotocol data units from said second sequence of protocol data units tobe transmitted; concatenating the compressed data and the reconstructiondata to form a sequence of concatenated data; and slicing said sequenceof concatenated data into data blocks, the data field of each of saidprotocol data units to be transmitted comprising at least one of saiddata blocks.
 2. Method according to claim 1, further comprising thesteps of:receiving said first sequence of source protocol data units,data compression associating, with said protocol data units, acompressed data sub-field comprising data corresponding to thecompression of the data of tile data field of said source protocol dataunits, generating data for the reconstruction of said source protocolunits, concatenating said reconstruction data and said compressed datasubfield in said sequence of concatenated data, slicing of said sequenceof concatenated data into data blocks having a predetermined lengthchosen from a sub-set of at least one possible length, creating saidsecond sequence of protocol data units to be transmitted, the data fieldof each of said protocol data units to be transmitted comprising atleast one of said data blocks.
 3. Method according to claim 2, whereinsaid generating step comprises a step of associating with each of saidcompressed data sub-fields, a sub-header comprising at least oneinformation element representing the length of said compressed datasub-field.
 4. Method according to claim 1, wherein said reconstructiondata comprises at least one information element present in the headerfield of each of said source protocol data units.
 5. Method according toclaim 3, wherein said sub-header comprises at least one first bytebearing at least one information element present in the header field ofthe source protocol data unit corresponding to said sub-header, and atleast one second byte bearing the length of the associated compresseddata sub-field.
 6. Method according to claim 2, wherein said generatingstep comprises a step of associating with each of said compressed datasub-fields, a sub-header comprising at least one designator designatingthe position of the end of said sub-field in the data field of thecorresponding protocol unit to be transmitted.
 7. Method according toclaim 2, wherein said generating step comprises a step to generate amarker having a predetermined content, known to said receiver terminal.8. Method according to claim 2, wherein the generating step comprises astep of associating a descriptor of the content of the data field ofsaid protocol data unit with each protocol data unit to be transmitted.9. Method according to claim 1, wherein said compression algorithm is anadaptive compression algorithm managing dictionaries of series of dataat transmission and at reception, said dictionaries being updatedidentically at each transmission of one of said protocol data units. 10.Method according to claim 1, wherein said second associating stepcomprises the steps of:implementing a compression algorithm thatassociates a compressed data field with a source data field; comparingthe length of said source data field and compressed data field, andselecting the shortest data field as a compressed data sub-field. 11.Method according to claim 3, wherein said sub-header comprises aninformation element representing the source data field or the compresseddata field.
 12. Method according to claim 1, wherein said protocol dataunits belong to the group consisting of data packets, frames of data,and transport layer protocol data units.
 13. Method according to claim2, wherein said protocol data units have a protocol data unit lengthchosen from a set of at least one possible length, and in that thelength of said data blocks is a sub-multiple of said chosen length ofprotocol data units.
 14. Method according to claim 1, further comprisinga step of verifying that the receiver comprises means for carrying adecompression method corresponding to said compression method, beforeactivating a compression.
 15. Method according to claim 1, furthercomprising the step of providing a first time-delay before the creationof a non-full protocol data unit to be transmitted.
 16. Method accordingto claim 15, further comprising the step of providing a secondtime-delay before the creation of a protocol data unit to be transmittedcomprising at least one full data block of a predetermined length and nodata block that is not full, the second time delay being smaller thanthe first time delay.
 17. Method for the decompression of compressedprotocol data units according to the method of claim 1, furthercomprising the steps of:searching for anti analyzing said data enablingthe reconstruction of said first sequence of protocol data units,decompressing said data corresponding, in compressed form, to the datacontained in the data fields of the source protocol data units; andrestituting said first sequence of source protocol data units.
 18. Datacompression and/or decompression device, of the type applied to a systemof data transmission by exchanging protocol data units between at leastone transmitter terminal and at least one receiver terminal, each ofsaid protocol data units comprising a header field and a data field,characterized in that it comprises compression means associating, in atleast one direction of data transmission and for at least onecommunication, with a first sequence of N source protocol data units, asecond sequence of M protocol data units to be transmitted, M beingsmaller than or equal to N, said compression means comprising:datacompression means, according to at least one compression algorithm,providing for the compression of data contained in the data fields of atleast two of said source protocol data units; means for generatingreconstruction data that enable the reconstruction of said firstsequence of source protocol data units from said second sequence ofprotocol data units to be transmitted; means for concatenating thecompressed data and the reconstruction data to form a sequence ofconcatenated data, and for slicing said sequence of concatenated datainto data blocks; and means for transmitting protocol data units to betransmitted comprising at least one of said data blocks.
 19. Deviceaccording to claim 18, further comprising means for decompression ofdata compressed by said compression means, said decompression meanscomprising:means for dividing said reconstruction data and said datablocks comprised in said protocol data units to be transmitted;decompression means symmetric to said data compression means, providingfor decompressed source data; and means for reconstruction of saidsource protocol data units, from said decompressed source data, underthe control of said reconstruction data.
 20. Device according to claim18, further comprising means of connection to at least onetransmitter/receiver terminal behaving similarly to the datatransmission system, and means of connection to said data transmissionsystem behaving similarly to said transmitter/receiver terminal. 21.Device according to claim 18, wherein said device is integrated with oneof said terminals or with an element of a communications networkinterconnecting said terminals.